Didymosphenia geminata (Lyngb.) M. Schmidt

Common Name: Didymo

Synonyms and Other Names:

Echinella geminata, Gomphonema geminatum, Didymosphenia geminatum [synonyms for the Great Lakes native strain]; Styllaria geminata, Dendrella geminata, Lyngbyea pulvinata var. geminata; Didymo

Copyright Info

Identification: Cells are distinguished by their large silica cell wall (frustule) shaped like a curved bottle (MISIN 2017). Cells feature a raphe, which is a structure used for movement on surfaces. Extracellular stalks that form nuisance blooms are secreted from the raphe (Spaulding and Elwell 2007). While microscopic, Didymosphenia geminata forms dense mats of extracellular stalks that are pale yellowish brown to white in color. Nuisance mats resemble fiberglass and have the texture of wet wool, but do not feel slimy to the touch ( Pennsylvania Sea Grant 2013).

Size: cells to 140 micrometers

Native Range: Northern Europe and Asia.  Possibly northern North America.  

Great Lakes Nonindigenous Occurrences: While the species is cryptogenic in origin, it has been observed throughout the Great Lakes region.  Prior to ~1990, all records of this form were as short-stalked unicells which did not form mats.   

Pre-1990 records and records of short-stalked unicells are presumed to be native are not included in the map or table.  These include: Great Lakes (Stoermer 1975, Stoermer & Kreis 1978, Stoermer et al. 1999), Lake Superior (Fox et al. 1967, 1969, 1973, Nelson et al. 1973, Stoermer 1980, 1993, Stoermer et al. 1986, Moffat 1994), Mackinaw Island (Bailey 1842), Lake Michigan (Briggs 1872, Thomas & Chase 1887, Marsh 1895, Ward 1896, Chase 1902, Britton 1944), Chicago (Boyer 1927), Indiana (Palmer 1930, Lindsey et al. 1969), New York State (Hohn 1951), Buffalo (Day 1882), Niagara River (Kützing 1849, De Toni 1891), and the Cayuga Lake Basin (Burkholder 1931) as well as Canadian records for Island of Montreal (Miller 1915), Ontario (Duthie 1973), and  Quebec (Bellavance 2006, Côté 2007, MDDEP-MRNF 2007, Blais 2008).


Table 1. Great Lakes region nonindigenous occurrences, the earliest and latest observations in each state/province, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Didymosphenia geminata are found here.

Full list of USGS occurrences

State/ProvinceFirst ObservedLast ObservedTotal HUCs with observations†HUCs with observations†
MI199420223Lake Superior; Manistee; St. Marys
VT201420141Winooski River

Table last updated 2/26/2024

† Populations may not be currently present.

Ecology: Didymosphenia geminata can be found in freshwater rivers, streams and lakes. This taxon prefers cool water of low conductivity (Patrick and Reimer 1975) and may be useful as indicators of increases in salt concentrations in the Great Lakes (E. F. Stoermer, personal communication).  The species is predominantly benthic but has been observed in planktonic samples (Blanco and Ector 2009). It prefers clear streams and rivers with shallow, moderately-flowing water with a pH of 7 or above and rocky substrate (MISIN 2017; Sea Grant Pennsylvania 2013; Spaulding and Elwell 2007). Historically the species was restricted to low-nutrient waters but has recently seen large range expansions reportedly occurring in eutrophic rivers, showing much greater tolerance for nutrient and flow conditions than previously expected. This may be attributed to a genetic variant with broader tolerances than the original species.  Nuisance benthic growths are only known to occur in flowing water.  While this species seems to be confined to cold areas, it reaches its highest biomass at higher water temperatures of ~20°C (Blanco and Ector 2009). It has also been observed at temperatures of 27°C (Spaulding and Elwell 2007).  

Didymosphenia geminata is capable of sexual reproduction, but recent nuisance blooms occurring globally have been a result of asexual vegetative cell division (Bothwell and Spaulding 2008). When the cell divides, the stalk divides as well (Spaulding and Elwell 2007).  Didymosphenia geminata typically blooms in streams with low to undetectable levels of phosphorous.  Short-term nitrogen and phosphorus additions followed by nutrient limitation have been reported to increase cell densities and division rates, possibly accelerating mat formation. The addition of nitrogen and/or phosphorus can stimulate cell division and increases the frequency of dividing Didymosphenia geminata cells (Bray et al. 2017; Jackson et al. 2016). As nutrient  concentrations (particularly dissolved phosphorus) decrease, stalk production tends to increase  (Jackson et al. 2016).  Current research suggests that stalk production allows for better access to light in addition to providing habitat for nutrient cycling and phosphorus acquisition via phosphatase production (Bray et al. 2017).

Didymosphenia geminata is a food source for macroinvertebrates (Spaulding and Elwell 2007). Mats formed in blooms create microenvironments that can be colonized by diatoms and other organisms (Blanco and Ector 2009). Recent studies on blooms have revealed supersaturated concentrations of dissolved oxygen in D. geminata mats, suggesting that other photosynthetic organisms exist within these mats, allowing Didymosphenia geminata to compete with other algae for both nutrients and light (Blanco and Ector 2009; Spaulding and Elwell 2007).

Great Lakes Means of Introduction: Ample opportunities exist by which non-native strains of Didymosphenia geminata may have been introduced to the Great Lakes.  Didymosphenia geminata has been shown to survive outside of the stream environment. Cells are able to survive and remain viable for 40 days in cool, dark, damp conditions. Angling equipment, boot tops, neoprene waders, and felt-soles provide a particularly suitable environment for cells to remain viable. Cells can hitchhike on this equipment and other recreational equipment into new bodies of water (Spaulding and Elwell 2007). Freshwater diatoms are dispersed through the flow of water and transport by other organisms, primarily waterfowl (Kristiansen 1996).

Great Lakes Status: Cryptogenic in the Great Lakes.   Didymosphenia geminata was historically considered native to the Great Lakes (e.g., Stoermer 1965 collection, Stoermer 1975). However, prior to 1990, all records of this species in the Great Lakes were as short-stalked unicells which did not form mats and the species was considered relatively rare in the region. It remains uncertain (West et al 2020) whether the expanding population of ‘rock snot’ is a separate strain (Bothwell et al 2006) or a change in growth triggered by environmental factors (Bothwell et al 2014) or entirely separate mechanisms.

Great Lakes Impacts: Didymosphenia geminata has a high environmental impact in the Great Lakes.

Blooms of Didymosphenia geminata form mats which can be over 20 cm thick. Extracellular stalks trap fine sediment, changing the nature of substrate and have potential long lasting effects due to the apparent resistance of stalks to degradation by bacteria and fungi (Spaulding and Elwell 2007).  These cause complex changes to the water chemistry including dissolved oxygen(Blanco and Ector 2009, Bhatt et al. 2008, Spaulding and Elwell 2007).  Mats are capable of engulfing the stream bottom, smothering native species of plants, insects, mollusks, and algae, and reducing habitat for insects for aquatic insects and fish (Kilroy 2004, Spaulding and Elwell 2007, Larson, 2007; Larson and Carreiro, 2008, Kilroy et al. 2009, Gillis and Chalifour 2010, PPereira and Perez 2019, MISIN 2023).  Fish reproduction may be directly impacted by reduction of suitable spawning areas (Blanco and Ector 2009).  Additionally, Didymosphenia geminata mats may provide habitat for Tubifex tubifex, which is a host to whirling disease (Myxobolus cerebralis) (Byle 2014).

Didymosphenia geminata has a high socioeconomic  impact in the Great Lakes.

Didymosphenia geminata is frequently reported by homeowners, recreationalists and land managers to be unsightly; growths are frequently mistaken for raw sewage (Spaulding and Elwell 2007, WI DNR 2023 citing Campbell 2005). D. geminata makes recreation, particularly swimming and fishing, unpleasant. This species also fouls fishing gear (MI DNR 2023) and as a result has significant negative impacts to tourism and recreation (WIDNR 2023). Diminished aesthetic value can impact tourism revenue generated by outdoor recreation and thick mats making fly-fishing difficult (Spaulding and Elwell 2007). D. geminata have the potential to impact trout fisheries by altering the aquatic invertebrate food base (Pennsylvania Sea Grant 2013) by reduction of suitable spawning areas (Blanco and Ector 2009), and by providing habitat which increases prevalence of whirling disease (Byle 2014). Didymosphenia geminata thrives in and clogs canal systems which are used for water transport for hydropower, agriculture, aquaculture, and human consumption (Spaulding and Elwell 2007, Pennsylvania Sea Grant 2013).

There is little or no evidence to support that Didymosphenia geminata has significant beneficial impact in the Great Lakes.
Biomedical applications for the polysaccharide stalks are in early stages of development (Somanader et al 2022)


Management: Regulations

Jurisdiction Regulation Law Description Date Effective
Illinois Other 515 ILCS 5/20-90 This species is not on the Illinois Aquatic Life Approved Species List and if it is not otherwise native to Illinois it is illegal to be imported or possessed alive without a permit. 7/9/2015
New York Prohibited 6 NYCRR Part 575 It is prohibited in New York and cannot be knowingly possessed with the intent to sell, import, purchase, transport or introduce nor can any of these actions be taken. 3/10/2015
Pennsylvania Other NA This species is listed as invasive in Pennsylvania, however, no specific regulations are defined. NA
Wisconsin Prohibited Chapter NR 40, Wis. Adm. Code It is a prohibited species in Wisconsin and one cannot transport, possess, transfer, or introduce this species without a permit. 4/1/2017

Table last updated 7/05/2022. Always check federal, state/provincial, tribal and local regulations directly for the most up-to-date information.



There are no known biological control methods for this species.

Managers of canal systems implement regular removals of Didymosphenia geminata by scraping growths from the concrete surfaces (Spaulding and Elwell 2007).  Flushing flows like those used for channel maintenance have been shown to reduce D. geminata benthic mats in the Rocky Mountains, Canada (Cullis et al., 2015).

The spread of Didymosphenia geminata can be reduced by cleaning boots, waders, boats, float tubes, and angling gear before traveling between bodies of water. Decontamination of gear has been shown to effectively destroy cells of Didymosphenia geminata. Heating for prolonged periods or freezing for several days can sterilize gear. Gear can also be soaked in hot water (140°F) for one minute to sanitize. Education and outreach related to this procedure could be implemented to minimize the spread of the species (Hoddle 2017; Sea Grant Pennsylvania 2013; Spaulding and Elwell 2007).

Absorbent items require longer soak times to properly sanitize. Items should be soaked for at least 40 minutes in hot water above 140°F for 30 minutes if the hot water is kept above 115°F is mixed with 5% dishwashing detergent (Sea Grant Pennsylvania 2013).

Investigation of ten potential control agents (algaecides/biocides) was initiated through experimental trials (Jellyman et al., 2006). The control agents were rated based on their effectiveness on causing cell mortality and degradation of biomass. Trials in artificial channels showed that a chelated copper compound was the most effective in killing D. geminata cells and minimizing effects on non-target species (Clearwater et al., 2007a).

Low phosphate levels are associated with bloom formation, enrichment with slow- and quick-release fertilizers was tested in a river in South Dakota, USA, and reduced D. geminata biomass locally (James et al., 2015).

Recreational gear can be sanitized by soaking gear in a 2% bleach solution, 5% salt solution, antiseptic hand cleaner, or dishwashing detergent for one minute (Sea Grant Pennsylvania 2013).

Remarks: Records from the Great Lakes region prior to 1990 or that are unicells with short stalks rather than mat-forming colonies are not included in the mapping data as they are presumed to be native.  

References (click for full reference list)

Author: Sturtevant, R., K. Hopper, and C.R. Morningstar

Contributing Agencies:

Revision Date: 1/15/2024

Citation for this information:
Sturtevant, R., K. Hopper, and C.R. Morningstar, 2024, Didymosphenia geminata (Lyngb.) M. Schmidt: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, and NOAA Great Lakes Aquatic Nonindigenous Species Information System, Ann Arbor, MI, https://nas.er.usgs.gov/queries/greatLakes/FactSheet.aspx?Species_ID=2856&Potential=N&Type=0&HUCNumber=DGreatLakes, Revision Date: 1/15/2024, Access Date: 2/26/2024

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.